The invention relates to a camshaft for an internal combustion engine having a cam disk support shaft, to which a plurality of cam disks and one drive wheel are attached, via matching outer cam disk support shaft and cam disk opening profiles. The invention also relates to a method for producing a camshaft, in which method a plurality of cam disks and at least one drive wheel are attached to a cam disk support shaft.
Camshafts of the type, which comprise a plurality of individual parts and are used in internal combustion engines to control the valve opening times, are called constructed camshafts.
DE 42 09 153 C2 discloses a profile for a detachable shaft/hub connection, in which profile both the shaft and the hub comprise more than one curved wedge corresponding to a logarithmic spiral function and have a micro-toothing at the circumference. Such a multi-wedge profile has the disadvantage that there are circumferential regions which are not in contact and the radial profile extent is very small. The gradient of the logarithmic spirals is selected such that the shaft/hub connection is maintained substantially by means of non-positive locking.
In the camshaft described in DE 41 21 951 C1, the cam disks are produced by means of forging and can have an opening whose shape deviates from that of a circle. The shaft profile is shaped by means of rolling and has elevations and depressions which lead to circumferential and therefore circular cross-sectional variations on the shaft. Circumferential channels are formed in the shaft by means of rolling, said channels pushing out circumferential beads as a result of material displacement. The radially elevated portions extend annularly in the axial direction of the cam disk support shaft and are oversized in the radial direction in relation to the openings in the cam disks. During joining, the cam disks are individually pressed, in a similar way to a longitudinal interference fit, onto the support shaft in the axial direction, between two profiling operations in each case. In addition, the cam disks must have a chamfer at the edge, so that they do not twist as they are pressed onto the shaft. A disadvantage of this is that when a cam disk is pressed onto the support shaft, the shaft beads are partially smoothed and lose their oversize because of wear during joining. For this reason, in the described method, the cam disks are reamed, in an expensive fashion, in order to form a rotationally symmetrical micro-toothing.
The so-called internal high-pressure forming method, which can result in considerable cost savings, is often used in the production of other camshafts. The main advantage of the constructed camshaft produced by internal high-pressure forming over conventional solutions is a reduction in material costs. A relatively cheap, unprocessed steel material is used for the actual shaft, which is also called a cam disk support tube, and high-grade, alloyed, hardenable ball bearing steel is used for the cam disks. The support tube is upset, in order to increase the wall thickness, at that end at which the camshaft drive wheel is attached. The support tube is machined at the ends and at its circumference. The cam disks are forged, are pre-machined by cutting and are heat treated. After joining by means of internal high-pressure forming, the cam shapes and the camshaft bearing seats are ground on the assembled camshaft in different work piece fixture mounting positions.
However, camshafts for commercial vehicles must be capable of transmitting significantly greater torques than camshafts for passenger automobiles. The reasons for this are the higher gas exchange valve operating forces as a result of the greater piston displacement volumes. In addition, commercial vehicle engines are sometimes used, in special applications, for driving auxiliary units by way of the camshaft, for example in agricultural machines for driving hydraulic units by way of the camshaft.
In this context, the internal high pressure forming process is considerably restricted: it necessitates a hollow camshaft, or a tube for holding the cam disks, whose wall thickness additionally cannot be so large, so that the required expansion pressures can still be provided. As a result, the construction material for the camshaft tube is generally relatively expensive. In the relevant diameter ranges, seamlessly drawn or longitudinally welded tubes which may be used are more expensive than the rolled solid round material. Here, it must be taken into consideration that, for reasons of strength, the tube must be deformed at one end, and that a closure cover is required at one end in order to prevent oil from flowing out. A second aspect is that the tube has a lower resistance with regard to torsional and bending loads than the solid shaft, requiring, under some circumstances, and for comparable loading, that the shaft tube is of greater diameter.
The internal high pressure forming technique also requires a relatively high operating system investment. On the one hand, this is because of the hydraulic unit required for generating the required pressure, and on the other hand, there are safety-related requirements which influence the system costs on account of the very high operating pressures of 2500 to 3000 bar. A further negative cost aspect of the internal high pressure forming method is the day-to-day operating costs. The seals which seal off the internal high pressure forming lance from the camshaft tube are subject to considerable wear and must be exchanged regularly, which in turn limits the degree of equipment efficiency of the system. As a result of the non-positive transmission of the operating forces, it is also only possible to a limited extent for the internal high pressure forming technique to be an operationally reliable non-positive shaft/hub connection for commercial vehicle camshafts.
In order for it to be at all possible, to join the cam disks to the support tube, in known constructed camshafts, the respective cam disk bore must be pre-machined. This can only be done in the unhardened state of the shaft. In order to provide the cam disks with their final hardness, it is necessary to heat them, normally by induction heating, and subsequently to quench them in a water bath or oil bath.
One method for producing a constructed camshaft using internal high pressure forming, and a constructed camshaft made from a shaft tube and elements which are pushed onto the tube, is known from EP 0 265 663 B2. The shaft is expanded hydraulically, resulting in the shaft/hub connection being generated by means of non-positive interlocking.
In accordance with EP 0 328 009 B1 or EP 0 328 010 B1, a tube can also be expanded by means of internal high pressure after assembly of the shaft, whereby the cam disks are fastened to two tubes, which are placed over one another, in order to increase strength. Torque is transmitted in a non-positively locking manner. This solution, however, is relatively expensive because of the plurality of components required.
EP 0 374 389 B1 discloses a method for pre-treating components of an constructed camshaft. The document describes heat treatment measures for a tube which are intended to facilitate expansion of the tube as a result of internal high pressure shaping or to provide for the bearing location a greater degree of hardness.
In the constructed shaft according to EP 0 374 394 B1, the cam disk support tube is pre-formed with different cross sections, so that during the subsequent expansion by means of internal high pressure forming, only those tube sections which hold the cam disks are plastically deformed. The tube sections between the individual cam disks are only expanded elastically.
In the method described in EP 0 313 565 B1 for producing a camshaft, tubes are used as supports for the cam disks. The cam disks are shaped in a die together with the support tube starting from a circular cross section, so that a constructed camshaft is produced. It is a disadvantage that hardened cam disks cannot be shaped at room temperature, since the cam disks would otherwise break apart or at least cracks could form in them. A separate heat treatment process is therefore required in the method described in this document.
One method for producing a constructed camshaft using internal high pressure deforming, and a constructed camshaft comprising a shaft tube and elements which are pushed onto the shaft tube, is described in EP 0 265 663 A1. The cam disks can have inner profiles in their openings in order to obtain positive locking in addition to the non-positive locking, the tube which forms the shaft being plastically deformed while the cam disks are elastically expanded.
A constructed camshaft comprising a hollow shaft which is produced by means of internal high pressure deforming is known from EP 0 516 946 B1. The cam disks which are fastened to the shaft have a circular cross-section and a groove which extends in the axial direction. The groove is at least partially filled with the shaft material as a result of plastic deformation of the shaft during the internal high pressure deforming process, resulting in a positively locking rotational connection.
A method for producing a single-piece hollow camshaft is described in EP 0 730 705 B1, in which method a tube is expanded in a die by means of internal high pressure deforming in such a way that a hollow camshaft is produced. This has the advantage that no separate cam disks need be produced. On the other hand, it is a disadvantage that heat treatment of the camshaft is necessary. In addition, the wall thickness of the camshaft is reduced to a particularly high degree in the regions of the cam peaks, as a result of which it is barely possible to meet the strength requirements for a commercial vehicle camshaft using this technology.
A camshaft and a method for producing the same are described also in EP 0 970 293 B1. Here, thin cam disks are punched out of a metal sheet or out of a sheet metal strip. A plurality of thin cam disks is assembled on top of one another or adjacent to one another to form sheet metal stacks. Accordingly, a cam disk is composed of a plurality of parts which are ultimately joined to a tube by means of internal high pressure deforming of the shaft. On the circumference, the cam disks can have a toothing or a notch-like profile which serves to provide for rotational locking.
The constructed camshaft known from EP 0 856 642 A1 is based on a longitudinal interference fit, the joining partners being coated at the joining points. The coating can be a phosphate layer or else adhesive. A profiling option which is not specifically described is also claimed.
EP 0 839 990 B1 proceeds from a cam disk support shaft produced by means of casting. This support shaft can be profiled at the points where the cam disks are fastened. The rotationally-non-symmetrical profile may be formed integrally during casting for the purpose of balancing and therefore serves to provide for a better mass distribution of the shaft. The cold forming of cast-iron components however is generally problematic because of their brittleness.
In the very similar method according to DE 37 17 190 C2, the profiling is carried out by means of rolling rods which have longitudinal grooves. The profile of a groove in the rolling die substantially follows the movement direction of the rolling rod, which has the same profile in every cross section perpendicular to the movement direction. In the case of a thread-like, bead-shaped enlarged portion of the cam disk support shaft, if such an enlarged portion is provided, the depression does not extend exactly in the movement direction of the rolling rod, but is inclined by a thread gradient angle. The cross sections of a rolling rod are then not entirely equal over the length. When viewed from the side, however, every rolling rod appears as being rectangular. The longitudinally extending straight delimiting lines of the side view of a rolling rod are parallel to the movement direction. The purpose of the polygon claimed here as an example of the support shaft profile, whose shape deviates from that of a circle, is intended to approximate a circle.
The constructed camshaft described in DE 195 20 306 C1 involves an indirect positively locking connection. A corrugated clamping sleeve is used which engages a rotationally symmetrical shaft toothing and an inner toothing, which is likewise rotationally symmetrically formed at the cam disk opening. A disadvantage of this, however, is the need for handling the clamping sleeve as a separate component.
EP 0 580 200 B1 relates to the cam disk being designed so as to be of a lightweight construction, for which purpose said cam disk is produced from a thin metal sheet. This design, however, is hardly capable of meeting the strength requirements of a camshaft.
The formation of an axial transition zone between two closely adjacent raised cam portions is known from EP 0 459 466 B1. This is significant only for single-piece camshafts but not for constructed camshafts.
In the method proposed in EP 0 650 550 B1, a cam disk support tube is expanded mechanically by means of a mandrel which is pushed or pulled through a disk support tube. It is additionally required that the cam disk support tube has different wall thicknesses before joining. The joining face can have recesses, pockets or a toothing. If a toothing is provided for the profile of the joining face, such toothing is to be formed on both joining partners, that is to say on the camshaft and on the cam disks.
In the very similar method according to EP 0 663 248 B1, wall thicknesses which are different in terms of shaping are formed in a tube by means of a stepped mandrel.
A constructed crankshaft and a method for producing the same are described in DE 100 61 042 C2. A conical curved wedge is used herein. A maximum of two crank webs can be joined to a crankpin journal by means of rotation relative to one another. The joining faces must be machined by cutting on account of the stringent tolerance requirements. During joining, the shaft is substantially elastically deformed, with the connection being releasable.
All the methods described are therefore incapable of meeting the demands on a highly-loaded camshaft. There is no solution which can be implemented in a simple and cost-effective manner.
It is therefore the principal object of the present invention to produce a relatively inexpensive camshaft which fulfills the high strength requirements, and a method for producing such a camshaft which method can be carried out with relatively little outlay and therefore at low costs.